Stone pressure plates unusable

I placed a stone pressure plate down in front of a iron door to find that stepping on the plate does not open the door as it should. Wooden pressure plates work with wooden and iron doors. In fact, when stepping on the stone pressure plate when it is not connected to anything (just sitting on the ground), it appears to simply not be activating. In this same situation, the wooden pressure plate again continues to work normally. I have screenshots as well.